1. Field of the Invention
The present invention relates to apparatus and methods for detecting the presence of water in a petroleum fluid.
2. Description of the Prior Art
Electrical power transformers of the size and capacity used by commercial power utilities to maintain primary transmission line voltage are normally charged with a petroleum fluid called insulating or dielectric oil for the dual purpose of insulating the primary and secondary transformer windings, each from the other, and also as a cooling medium to remove inductance heat from the proximity of the transformer core and windings.
Such dielectric oil is also used in relays, switches, oil circuit breakers, and other large capacity power transmission equipment. For brevity, the following description will hereafter be related to transformers, but it will be understood that all such oil insulated transmission devices are included.
In operation, the oil reservoir of such transformers is sealed from the atmosphere. However, if the unit is to be moved from one operating location to another, it is often necessary to remove the oil for weight reduction.
Oil is also removed from a transformer periodically to filter an accumulation of colloidally dispersed carbon particles from the fluid which develop due to localized overheating of the hydrocarbon substance in the course of operation.
It is during these occasions of field removal and recharging that opportunity is given for moisture infusion into the system.
The primary objection to moisture in the oil charge is that water concentrations above a very low threshold percentage reduce the dielectric characteristic of the oil (increase the conductance) sufficiently to permit arcing across the windings. Since the e.m.f. between the windings may be in the order of 1000 v or more, once the arc begins, it is sustained and intense. Consequently, rapid thermal dissociation of the hydrocarbon compounds occurs to generate catastrophic pressure increases within the sealed transformer housing. Transformer explosion is the terminal result.
Although great care is normally taken during the oil charging and cleaning process, precautionary procedures are, nevertheless, subject to occasional human failure. As a result of such occasional failures and the enormous expense for repair or replacement of the damaged equipment, the consequent economic burden is a significant factor in the operating overhead of utility companies.
One of such precautionary procedures practiced universally is a final, full-flow filtering of the oil as it is returned to the transmission equipment reservoir. Presently, two types of such filters are predominately used.
One type of such filters is a laminated plate assembly wherein a series of perforated steel plates are separated by sheets of "blotter paper." A specification of such paper may be 25 mil thickness and approximately 26 pounds per cubic foot density. As specified by one manufacturer of such paper, the Eaton, Dikeman Co. of Mt. Holly Springs, Pa. 17065, the paper is 25 caliper, 75 pounds per 500, 20 in. .times. 20 in. sheets. This paper is believed to be substantially pure, bleached cellulose except for the inclusion of less than 0.1% melamine formaldehyde. The controlling characteristics of this blotter paper for the present purposes is the high permeability and affinity for water thereof. The fact that the fiber surfaces may be completely surrounded by oil as in the case of complete immersion, does not diminish the capacity of the fiber to absorb water coming into contact therewith.
The foregoing description of such blotter paper will hereafter serve as a definition of the term "hydrophilic material" and permeations thereof. There are other manufacturers of suitable blotter paper and the thickness and density may vary over a wide range. However, the characteristic capacity to absorb water in the immersed presence of oil is critical. An electrical characteristic of this hydrophilic material critical to the present invention will subsequently be described.
Another filter type widely used by the utility industry for charging transformers and relays is a cartridge type. The basic construction of such cartridge filters comprises inner and outer perforated cylindrical sleeves separated by a packed annulus of hydrophilic filter material. In this case the filter material is substantially less compacted than that of the machine laid sheet described above but the flow passage (radial) thickness is substantially greater. Consequently, the absolute fiber quantity contacted by the oil as it flows radially outward from the inside bore of the interior sleeve and across the filter annulus to the exterior sleeve may be at least as great or greater.
Although there are many devices and techniques for separating water from oil, the aforedescribed filters are distinct in their capacity to absorb and retain small quantities of water that is distributed about the oil mass in isolated pockets or discrete droplets. However, such filters are also subject to saturation of this capacity. Such a water saturated filter is useless. Unfortunately, operator knowledge of such saturation is quite another matter. Consequently, filter elements are replaced on a routine schedule based on volumetric throughput of oil. The cost of maintenance labor and filter element replacement contribute to balance a probability equation which dictates maintenance frequency. In operation, however, such filters are usually replaced when only 30% or less saturated. Nevertheless, occasionally a fresh filter becomes saturated in the process of a single charge thereby permitting sufficient water to pass into the equipment to cause damage. Prior to the present invention, no device or procedure was available to positively prevent such occurrences.